1. Field of the Invention
This invention relates to a method for adjusting positions of a plurality of radiation images, which are to be subjected to superposition processing or subtraction processing, by eliminating shifts in positions among the radiation images. This invention particularly relates to a method for adjusting positions of radiation images, wherein image patterns of a marker for position adjustment need not be embedded in the radiation images.
2. Description of the Prior Art
It has heretofore been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation, which carries image information of an object, such as a human body. In this manner, a radiation image of the object is stored on the stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, when it is exposed to the stimulating rays, is photoelectrically detected and converted into an electric image signal. The electric image signal is then processed, and the processed image signal is then used during the reproduction of a visible image, which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness. The visible image finally obtained may be reproduced in the form of a hard copy or may be displayed on a display device, such as a cathode ray tube (CRT) display device.
Techniques for carrying out superposition processing on radiation images have heretofore been disclosed in, for example, U.S. Pat. No. 4,356,398. In general, radiation images are used for diagnoses of illnesses and for other purposes. When a radiation image is used for such purposes, it is required that even small differences in the radiation energy absorption characteristics among structures of an object can be detected accurately in the radiation image. The extent, to which such differences in the radiation energy absorption characteristics can be detected in a radiation image, is referred to as the contrast detection performance or simply as the detection performance. A radiation image having better detection performance has better image quality and can serve as a more effective tool in, particularly, the efficient and accurate diagnosis of an illness. Therefore, in order for the image quality to be improved, it is desirable that the detection performance of the radiation image may be improved. The detection performance is adversely affected by various noises.
Superposition processing is carried out in order to reduce the aforesaid noises markedly so that even small differences in the radiation energy absorption characteristics among structures of an object can be found accurately in a visible radiation image, which is reproduced finally, i.e. the detection performance of the radiation image can be improved markedly. Specifically, a radiation image is stored on each of a plurality of stimulable phosphor sheets, which have been placed one upon another. Thereafter, an image read-out operation is carried out for each of the stimulable phosphor sheets. A plurality of image signals, which have been obtained from the image read-out operations, are added to one another. In this manner, various noises described above can be reduced.
By way of example, when superposition processing is to be carried out, two stimulable phosphor sheets have heretofore been housed in a cassette such that they may overlap one upon the other. Radiation images of an object are then recorded on the two stimulable phosphor sheets housed in the cassette. Thereafter, an image read-out operation is carried out on each of the two stimulable phosphor sheets, and two image signals are thereby obtained. The two image signals are then added to each other.
Also, techniques for carrying out subtraction processing on radiation images have heretofore been known. When subtraction processing is to be carried out, two radiation images recorded under different conditions are photoelectrically read out, and digital image signals which represent the radiation images are thereby obtained. The image signal components of the digital image signals, which represent corresponding picture elements in the radiation images, are then subtracted from each other, and a difference signal is thereby obtained which represents the image of a specific structure or part of the object represented by the radiation images. With the subtraction processing method, two digital image signals are subtracted from each other in order to obtain a difference signal, and the radiation image of a specific structure can be reproduced from the difference signal.
Basically, subtraction processing is carried out with either the so-called temporal (time difference) subtraction processing method or the so-called energy subtraction processing method. In the former method, in order to extract the image of a specific structure of an object from the image of the entire object, the image signal representing a radiation image obtained without injection of contrast media is subtracted from the image signal representing a radiation image in which the image of the specific structure of the object is enhanced by the injection of contrast media. In the latter method, an object is exposed to several kinds of radiation having different energy distributions. Alternatively, the energy distribution of the radiation carrying image information of an object, is changed after it has been irradiated onto one of at least two radiation image recording media, after which the radiation impinges upon the second radiation image recording medium. In this manner, at least two radiation images, in which different images of a specific structure of the object are embedded, are obtained. Thereafter, the image signals representing at least two radiation images are weighted appropriately, when necessary, and subjected to a subtraction process, and the image of the specific structure of the object is thereby extracted.
Subtraction processing is extremely effective, particularly for medical diagnosis, and electronics research has continued to develop improved subtraction processing methods.
However, the problems described below are encountered in the superposition processing and the subtraction processing of radiation images, wherein stimulable phosphor sheets are utilized.
Specifically, when each of the superposition processing method and the subtraction processing method utilizing the stimulable phosphor sheets is to be carried out, at least two stimulable phosphor sheets are inserted into an image recording apparatus one after the other or simultaneously, and radiation images to be subjected to the superposition processing or the subtraction processing are recorded on the stimulable phosphor sheets. Thereafter, each of the stimulable phosphor sheets is inserted into an image read-out apparatus and exposed to stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by each stimulable phosphor sheet is detected, and the radiation image stored on the stimulable phosphor sheet is thereby read out. In such cases, even if the operations for recording and reading out the radiation images are carried out very carefully, a shift and a rotation will occur between the images to be subjected to the superposition processing or the subtraction processing. As a result, in the superposition processing, even if various noises are averaged and reduced, the entire area of the superposition image, which is obtained from the superposition processing, particularly edges of a structure in the superposition image, will become unsharp. Therefore, a superposition image cannot be obtained which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness. Also, in the subtraction processing, as a result of the shift and the rotation occurring between the images to be subjected to the subtraction processing, an image pattern to be erased in a subtraction image, which is obtained from the subtraction processing, cannot be erased. Alternatively, an image pattern to be formed in the subtraction image will be erased, and an artifact will occur. Therefore, an accurate subtraction image cannot be obtained. In this manner, the shift and the rotation occurring between the images to be subjected to the superposition processing or the subtraction processing adversely affect the image quality of the image obtained from the superposition processing or the subtraction processing.
The radiation image is stored as a latent image on the stimulable phosphor sheet and cannot be viewed directly like an X-ray image recorded as a visible image on X-ray photographic film. Therefore, the positions of two or more radiation images stored on the stimulable phosphor sheets cannot be visually matched to each other. Accordingly, if the shift and the rotation occur between the radiation images stored on the stimulable phosphor sheets, the shift and the rotation cannot be eliminated easily.
Also, even if the shift and the rotation between two radiation images can be detected by some means, considerable time will be required for conventional operations to be carried out in order to correct the image signals detected from the radiation images, particularly in order to eliminate the rotation between the radiation images. This is a very real problem in practical use.
In U.S. Pat. No. 4,710,875, the applicant proposed a subtraction processing method for radiation images, wherein a marker having a shape such that it may provide a reference point or a reference line is utilized. With the proposed method, image patterns of the marker are recorded on two stimulable phosphor sheets such that the patterns of the marker may be located at positions fixed with respect to radiation images stored on the stimulable phosphor sheets. When the radiation images are read out from the stimulable phosphor sheets, the patterns of the marker are detected. The amounts of a shift and a rotation between the two radiation images are then calculated with reference to the patterns of the marker. Thereafter, either one of the radiation images to be subjected to subtraction processing is digitally rotated and/or translated in accordance with the calculated amount of the rotation and/or the calculated amount of the shift. The image signal components of the image signals, which represent corresponding picture elements in the radiation images, are then subtracted from each other. The position adjusting step, which is carried out in the subtraction processing method for radiation images utilizing the marker, can also be applied to the aforesaid superposition processing method. In such cases, after the positions of the radiation images are digitally matched to each other, the image signal components of the image signals, which represent corresponding picture elements in the radiation images, may be added to each other.
However, with the proposed method, each time a radiation image of an object is recorded on a stimulable phosphor sheet, the pattern of the marker must be recorded together with the object image on the stimulable phosphor sheet. Also, the problems occur in that the image information of the object cannot be obtained from the portion of the radiation image stored on the stimulable phosphor sheet, which portion overlaps upon the position of the pattern of the marker.
Accordingly, in U.S. patent application Ser. No. 08/158,875 (see U.S. Pat. No. 5,623,560), the applicant proposed a method for adjusting positions of radiation images, wherein a marker, or the like, need not be used for position adjustment. The proposed method comprises the steps of (a) setting template regions on one of a plurality of radiation images, the positions of which are to be adjusted, (b) carrying out template matching on the other radiation images by using the template regions, (c) thereby obtaining at least two corresponding points in each of the plurality of the radiation images, and (d) carrying out affine transformation on the corresponding points such that the corresponding points in the plurality of the radiation images may coincide with one another. With the affine transformation, the correction with the rotating operation, the correction with the enlargement or reduction factor, and the correction with the parallel translation are carried out on the plurality of the radiation images.
With the proposed method for adjusting positions of radiation images, an image pattern of a marker, or the like, need not be recorded for position adjustment together with an object image in each of radiation images, and the positions of radiation images can be quickly and accurately matched to each other.
However, for example, as illustrated in FIG. 10 which is an explanatory view showing shift between radiation images, it often occurs that errors xcex94A and xcex94B of points A and B, which are among three corresponding points A, B, and C, with respect to reference corresponding points are small, and an error xcex94C of the point C, which is located at a position spaced far apart from the points A and B, with respect to a reference corresponding point is large. In such cases, if only two points A and B are obtained as the corresponding points, which are to be subjected to the affine transformation, the point C will not be transformed with the affine transformation. Therefore, the error xcex94C remains large, and an accurate position adjustment cannot be carried out. Also, the error xcex94C of the point C is larger than the errors xcex94A and xcex94B of points A and B. Therefore, with the aforesaid method for adjusting positions of radiation images, the errors of the respective points cannot be compensated for uniformly during the affine transformation, and the position adjustment cannot be carried out accurately.
FIG. 8 is a graph showing the results of position adjustments carried out with a conventional method for adjusting positions of radiation images. In FIG. 8, the sum |R| of the magnitudes of the vectors of the aforesaid errors (error vectors) is plotted on the horizontal axis, and the maximum value max |Ri| of the error vectors is plotted on the vertical axis. Also, in FIG. 8, the results of the position adjustments carried out on 55 sets of radiation images are shown. By way of example, as illustrated in FIG. 8, with the aforesaid method for adjusting positions of radiation images, in cases where the reference vector is taken as |R|=4.5 and max|Ri|=1.00, the results of the position adjustments on 10 sets of radiation images did not fall within the range of the reference level.
The primary object of the present invention is to provide a method for adjusting positions of radiation images, wherein a marker, or the like, need not be used for position adjustment.
Another object of the present invention is to provide a method for adjusting positions of radiation images, wherein the positions of radiation images can be quickly and accurately matched to each other.
The present invention provides a first method for adjusting positions of radiation images, wherein the positions of a plurality of radiation images are matched to one another such that the radiation images may be subjected to superposition processing or subtraction processing,
the method comprising the steps of:
i) setting template regions on a single radiation image, which is among the plurality of the radiation images,
ii) carrying out template matching, with which the template regions are matched with the radiation images other than the single radiation image,
iii) thereby obtaining at least three corresponding points in each of the plurality of the radiation images,
iv) taking the corresponding points in a single radiation image, which is among the plurality of the radiation images, as reference corresponding points,
v) calculating factors of affine transformation with the method of least squares, the affine transformation being represented by the formula       (                            u                                      v                      )    =                    (                                            a                                      b                                                          c                                      d                                      )            ⁢              (                                            x                                                          y                                      )              +          (                                    e                                                f                              )      
wherein u and v represent the coordinates of the reference corresponding point, x and y represents the coordinates of the corresponding point to be transformed, a, b, c, and d are the factors representing correction with a rotating operation and correction with an enlargement or reduction factor, and e and f are the factors representing correction with parallel translation, and
vi) carrying out affine transformation, in which the calculated factors of affine transformation are used, and with which the values of coordinates of the corresponding points in the radiation images other than the single radiation image having the reference corresponding points are transformed into values of coordinates of the reference corresponding points such that the reference corresponding points and the corresponding points in the radiation images other than the single radiation image having the reference corresponding points may coincide with one another.
The present invention also provides a second method for adjusting positions of radiation images, wherein the first method for adjusting positions of radiation images in accordance with the present invention is modified such that the plurality of the radiation images are obtained by exposing a plurality of stimulable phosphor sheets superposed one upon another to radiation, which has been produced by a radiation source and has passed through an object, thereby storing a radiation image of the object on each of the stimulable phosphor sheets, thereafter exposing each of the stimulable phosphor sheets to stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation, and photoelectrically detecting the emitted light, and
the radiation image, which has been obtained from the stimulable phosphor sheet located at the position closest to the radiation source, is taken as the radiation image, in which the reference corresponding points are to be set.
With the template matching, in cases where the template regions are set in a single radiation image in the manner described above, the template regions are moved on the other radiation image, and the locations, which best match to the template regions, are thereby found. The points representing the locations which have thus been found give the coordinates of the corresponding points.
The degree of matching in the template matching may be evaluated with a correlation method or sequential similarity detection algorithms (hereinafter referred to as SSDA).
With the correlation method, the product of image signal components, which represent corresponding picture elements in the corresponding regions on the radiation images, is calculated. The sum of the products, which have thus been calculated for the picture elements in the corresponding regions on the radiation images, is then normalized, and the value obtained from the normalization (hereinafter referred to as the normalized value) is taken as the grade of superposition. During the normalization, the product (the square) of the image signal component, which represents each picture element in each region, is calculated, and the sum of the products, which have thus been calculated for the picture elements in each region, is then calculated. Thereafter, the product of the sums, which have thus been calculated for the corresponding regions on the radiation images, is calculated. The square root of the product, which has thus been obtained, is taken as the denominator for the aforesaid sum of the products of image signal components, which represent corresponding picture elements in the corresponding regions on the radiation images. In cases where the corresponding regions are completely superposed one upon the other, the product of the image signal components, which represent corresponding picture elements in the corresponding regions on the radiation images, may not become equal to the square of each of these image signal components due to noise, or the like, and therefore the normalized value may not become equal to 1. However, in such cases, the normalized value will take the maximum value which is closest to 1. Therefore, the template regions may be moved in various ways on the radiation image. When the template regions have been moved such that the aforesaid normalized value may become maximum, it may be considered that the superposition of the corresponding regions on the radiation images have been accomplished. However, the judgment as to whether the template regions have been or have not been moved such that the aforesaid normalized value may become maximum can be made only after all of the movements have been carried out. The correlation method is described in detail in, for example, xe2x80x9cAutomated Cloud Tracking Using Precisely Aligned Digital ATS Picturesxe2x80x9d by Smith, et al., ibid., Vol. c-21, pages 715-729, July 1972.
With the SSDA, the difference between the image signal components, which represent corresponding picture elements in the corresponding regions on the radiation images, is calculated. The sum of the absolute values of the differences, which have thus been calculated for the picture elements in the corresponding regions on the radiation images, is taken as the grade of superposition. In cases where the corresponding regions are completely superposed one upon the other, even if the sum (the residual) does not become equal to 0 due to noise, or the like, the residual will become minimum. Therefore, the template regions may be moved in various ways on the radiation image. When the template regions have been moved such that the residual may become minimum, it may be considered that the superposition of the corresponding regions on the radiation images have been accomplished. If the positions of the corresponding regions on the radiation images shift from each other, the residual will increase sharply during the addition of the absolute values of the differences between the image signal components, which represent corresponding picture elements in the corresponding regions on the radiation images. Therefore, with the SSDA, when the residual becomes larger than a certain threshold value during the addition, the addition is ceased, and next movement of the template regions is begun. With the SSDA, only the addition is carried out during the calculation. Also, in many cases, the calculation is ceased before it is carried out for all of the picture elements in the corresponding regions on the radiation images. Therefore, the calculation time can be kept short. The SSDA method is described in detail in, for example, xe2x80x9cA Class of Algorithms for Fast Digital Image Registrationxe2x80x9d by Barnea, et al., IEEE. Trans., Vol. c-21, pages 179-186, February 1972.
With the method for adjusting positions of radiation images in accordance with the present invention, at least three corresponding points are obtained in each of the plurality of the radiation images by carrying out the template matching. Therefore, the accuracy of the position adjustment can be kept higher than with the conventional method wherein the positions of two corresponding points are matched to each other. Also, the coordinates of the corresponding points are transformed into the coordinates of the reference corresponding points by carrying out the affine transformation wherein the factors of affine transformation calculated with the method of least squares are used. Therefore, the corresponding points in each image are transformed such that the error between every corresponding point in each image and the reference corresponding point may become smallest. Further, with the method for adjusting positions of radiation images in accordance with the present invention, the factors of affine transformation are calculated with the method of least squares. Therefore, even if the number of the corresponding points becomes large, every corresponding point can be transformed so as to become closer to the reference corresponding point.
When the affine transformation is carried out on a radiation image in the manner described above, the image quality of the transformed radiation image becomes bad to some extent due to interpolation of image signal components. Also, in cases where a plurality of radiation images are recorded on a plurality of stimulable phosphor sheets with a single, simultaneous exposure to the radiation (i.e., with the one-shot image recording operation), the image quality of the radiation image recorded on a stimulable phosphor sheet, which is located at a position remoter from the radiation source, becomes worse due to effects of scattered radiation, or the like. Therefore, in cases where a plurality of radiation images are recorded on a plurality of stimulable phosphor sheets with the one-shot image recording operation, and the radiation image, which has been obtained from the stimulable phosphor sheet located at the position closest to the radiation source, is taken as the radiation image, in which the reference corresponding points are to be set, the position adjustment can be carried out accurately while the image quality of the radiation image, which has been obtained from a stimulable phosphor sheet located at the position close to the radiation source, is being kept good.
As described above, with the method for adjusting positions of radiation images in accordance with the present invention, a marker, or the like, need not be used for position adjustment, and the positions of radiation images can be quickly and accurately matched to each other.